1. Field of the Invention
The present invention relates to sampling circuits for sampling analog signals, and in particular, sampling circuits which can be selectively configured to sample multiple types of analog signals, including "return-to-reference" and "non-return-to-reference" types of signals.
2. Description of the Related Art
Solid state image sensors are used in many imaging applications, such as scanners, digital cameras and video cameras. Two of the most widely used image sensors are charge coupled devices (CCD) and contact image sensors (CIS). The output image signals for a CCD sensor typically have a negative polarity, while the output image signals for a CIS sensor typically have a positive polarity. Furthermore, some sensors generate output signal waveforms of the type referred to as return-to-reference ("RTR"), i.e., where the output waveforms remain at the signal reference level (e.g., corresponding to the signal level for a black pixel) for a predetermined time interval during each pixel time period. For other types of sensors, however, the output signal waveform is of the type referred to as non-return-to-reference ("NRTR") where the output signal waveform does not necessarily go to the signal reference level during each pixel time period.
In those circuits where the sensor output signal goes to a reference level at every pixel, correlated double sampling ("CDS") is often used to reduce signal noise. With CDS, the difference between the reference level and the present signal level is sampled at every pixel, thereby causing low frequency input signal noise to be removed. However, in those applications where the sensor output signal does not return to the signal reference level at every pixel, CDS cannot be used. Accordingly, the difference between some preset reference voltage and the present signal level at the sensor output is sampled.
Accordingly, due to the widespread use of multiple types of image sensors which, in turn, produce multiple types of sensor output signal polarities and waveforms, it is desirable to have a sampling circuit for such analog signals which is selectively configurable and is, therefore, suitable for use in sampling all such types of analog image signals.